The present invention relates to a power supply circuit, a display driver, an electro-optical device, an electronic instrument, and a method of controlling a power supply circuit.
As a liquid crystal display (LCD) panel (display panel in a broad sense) used in an electronic instrument such as a portable telephone, a simple matrix type LCD panel and an active matrix type LCD panel using a switch element such as a thin film transistor (hereinafter abbreviated as “TFT”) have been known.
The simple matrix type LCD panel easily reduces power consumption in comparison with the active matrix type LCD panel. However, it is difficult to increase the number of colors and display a video in the simple matrix type LCD panel. The active matrix type LCD panel is suitable for increasing the number of colors and displaying a video. However, it is difficult to reduce power consumption of the active matrix type LCD panel.
In recent years, an increase in the number of colors and display of a video have been increasingly demanded for a portable electronic instrument such as a portable telephone in order to display a high-quality image. Therefore, the active matrix type LCD panel has been widely used instead of the simple matrix type LCD panel.
The simple matrix type LCD panel or the active matrix type LCD panel is driven so that the voltage applied to a liquid crystal forming a pixel is alternately changed. As such an alternating drive method, a line inversion drive and a field inversion drive (frame inversion drive) have been known. In the line inversion drive, the polarity of the voltage applied to the liquid crystal is reversed in scan line units. An N-line inversion drive is also known in which the line inversion drive is performed in units of two or more scan lines. In the field inversion drive, the polarity of the voltage applied to the liquid crystal is reversed in field (frame) units.
The voltage level applied to a pixel electrode forming a pixel can be decreased by changing a common electrode voltage (common voltage) supplied to a common electrode opposite to the pixel electrode corresponding to inversion drive timing.
The inversion drive increases power consumption since an electric charge is repeatedly charged and discharged. JP-A-2004-184840 discloses a technology of reducing power consumption by reutilizing an electric charge discharged from a data line of the LCD panel.
However, the pixel electrode, to which a data voltage supplied to the data line from a data driver is applied, is capacitively coupled with the common electrode. Therefore, the voltage level of the common electrode changes due to a change in the voltage supplied to the pixel electrode. A change in the voltage level of the common electrode causes deterioration of the image quality. Therefore, the power supply capability of a power supply circuit which supplies the common electrode voltage is determined taking into consideration the maximum value of the amount of electric charge which must be charged or discharged in order to prevent a change in the voltage level of the common electrode. Therefore, the power supply circuit unnecessarily consumes power when the power supply capability is not required.
The data driver which supplies the data voltage corresponding to grayscale data to the data line of the LCD panel may precharge the data line before supplying the data voltage to the data line. The voltage level of the data line can be promptly set at a desired data voltage by precharging the heavily-loaded data line, so that deterioration of the image quality can be prevented.
While deterioration of the image quality can be prevented by precharging the data line, the data voltage supplied to the data line from the data driver significantly affects current consumption during the data line precharge operation in the subsequent horizontal scan period. Specifically, the amount of the current consumption during the precharge operation in the subsequent horizontal scan period is increased or decreased depending on the data voltage in the preceding horizontal scan period. It was found that power consumption can be reduced by reducing the above-mentioned effect.